One of tags which are used by Hetalia fans.

The tag is put on works those obviously could be attracting Berwald.

This tag is put by viewers, and most of works with this tag show this character in them.

"-ホイホイ(hoi-hoi)" means that the thing is actually attractive to a certain thing/person.

It comes from a roach-trap product called "ゴキブリホイホイ(Gokiburi-Hoi-Hoi)" that is sold in Japan ... and that, you might understand why the term is used for such thing.